Many off-highway machines are driven by hydraulic motors mounted to planetary wheel drives. Some of these vehicles are capable of speeds of up to 35-40 miles/hour. Due to the large mass of these machines and their relatively high speed, braking is a very important function.
Braking is usually categorized in two ways: service dynamic braking and parking static braking. On many high-speed off-highway machines, the service braking is handled by a combination of back-driving the hydraulic system and actuating a disc-caliper system on the output of the planetary wheel drive. Parking braking is usually done with a multi-disc wet brake at the input of the planetary wheel drive. This brake is usually spring-applied and released with hydraulic charge pressure.
There are a few shortcomings of a disc-caliper service braking system on high-speed off-highway hydraulic machines. Due to the potential large momentum associated with these machines an output disc-caliper service brake needs to be very large which makes it a costly feature. An output disc-caliper service brake is exposed to the environment. The environment for many of these machines can be severe and highly corrosive to a brake disc. For low to moderate braking, the hydrostatic system is used almost exclusively. This prevents corrosion from being “wiped off” frequently by the caliper.
U.S. Pat. No. 7,493,992 to Rogers states in the abstract thereof, as follows: “gearbox having an integral wet brake assembly is provided to replace the prior gearbox assembly of an underground mining machine. The brake housing is configured and contoured to be nestingly received upon the gearbox housing and without intrusion upon the output drive flanges. The brake housing includes concentric parking and service brake pistons adapted to engage and disengage a brake disc stack provided within a cavity machined into an end plate of the gear box assembly. An intermediate shaft of the gearbox is provided with a hub to engage the rotary discs of the brake disc stack. The parking brake is normally engaged by a spring and released by hydraulic pressure, while the service brake is normally released by a spring and engaged by hydraulic pressure.”
U.S. Pat. No. 5,601,160 to Horsch states, in the abstract thereof, “a hydraulically actuated brake assembly capable of rendering both a service brake function and a park brake function to a rotatable shaft that passes through a stationary housing. The brake assembly includes a series of wetted rotatable and nonrotatable brake members arranged in a stacked and intermixed relation within the housing and arranged coaxially about the shall. The brake assembly further includes a service brake piston and a park brake piston for individually or conjointly applying a compressive force against the brake members thereby providing a braking action to the shaft and a park brake piston. The pistons are preferably arranged in axial alignment with each other and coaxially about the shaft. The service brake piston responds to fluid pressure being introduced into, a first fluid receiving cavity. The park brake piston is spring applied and is responsive to the fluid pressure introduced into a second fluid receiving cavity. The brake assembly further includes a brake release mechanism for normally urging the service brake piston into a released position wherein the piston is disposed a fixed distance from the brake members. The brake release mechanism is specifically structured and designed to maintain a fixed clearance between the brake piston and the brake members when the brake piston is in its released position and self-compensates for wear on the braking surfaces of the brake members.”